1. Field of the Invention
The present invention relates to a method of producing a phenol novolak resin having an ortho ratio of 30% or more.
2. Description of the Related Art
Phenol novolak resins are used in various fields such as molding materials, epoxy resin curing agents, photoresist resins, epoxy resin raw materials and the like. It is well known that the properties thereof, particularly the reactivity changes depending on the ratio of the ortho position of methylene bonds derived from raw material aldehyde, namely, the ortho ratio (“Phenol Resin”: pp. 48 to 52, 1987, published from Plastics Age). For example, it is known that when the ortho ratio increases, curing speed by an amine compound increases. In the case of a novolak epoxy resin obtained by epoxidizing a phenol novolak resin, it is also known that curing speed increases likewise. Thus, novolak resins having various ortho ratios are produced according to their use.
For example, it is known that, in a reaction of a phenol with formalin, those having an ortho ratio of around 20% can be produced by using p-toluenesulfonic acid as a catalyst, and those having an ortho ratio of around 40% can be produced by using oxalic acid as a catalyst. It is also known that those having an ortho ratio of over 60%, called high-ortho novolak, can be produced by using zinc acetate. Further, in the case of production of a phenol novolak resin having an ortho ratio of around 40%, a catalyst used is weaker acidic as compared with that used in producing a phenol novolak resin having an ortho ratio of around 20%, therefore, for improving reactivity, there are also tried methods of reacting longer time, conducting dehydration with increasing the temperature, and the like (JP-A No. 62-275121).
However, these methods of improving reactivity have a problem of low yield, and further, have an industrial problem that a drain such as distilled water, separated water and the like contains a large amount of raw material phenols. Such a phenol novolak resin is usually produced by reacting a phenol and formalin under a normal pressure (Phenol Resin: pp. 83 to 85, 1987, published by Plastic Age K. K.).
On the other hand, methods of reaction under pressure are also known. For example, proposed are methods of producing a high-ortho novolak resin by reaction under pressure using an organic solvent (Makromol. Chem. 182. 2973 (1981), JP-A No. 2000-264941, and the like).
When a phenol novolak resin is produced in industrial scale, a method of removing the heat of reaction becomes a problem. In the case of a reaction under normal pressure, cooling efficiency is poor and heat removal is difficult by cooling only with a jacket because of high viscosity of the reaction system and a glass lining vessel used for reaction under an acidic condition, therefore, the reaction is generally conducted with removing the heat by refluxing water or an organic solvent present in the system with using a condenser.
On the other hand, in case of a reaction under pressure, even if a condenser is equipped, reflux is hard to occur and removal of the heat of reaction is difficult probably due to the presence of an inert gas used at the time of charging raw materials. Therefore, there are conventionally adopted a method of reducing the charging amount of a raw materials, a method of conducting reaction for a long period of time with dropping aldehyde in portion wise, and the like, according to the cooling ability of a reaction vessel jacket. However, in the former method, the production amount per reaction vessel decreases, and in the latter method, the reaction time elongates, accordingly, both methods have a problem of poor productivity.